1. 20 TheStructuralEngineer Feature
›
September2016 Special concretes in tall buildings
Introduction
The demand for new residential and
commercial space is growing in the UK, but
prime development space in some of our
busiest cities is declining at a rapid rate. The
solution for many developers is multistorey
buildings.
Research indicates that over 263 towers
are proposed in London alone over the next
10 years1
. Nearly half of these have been
granted planning permission already and, if all
are constructed, London will move from 48th
to fourth in the top 100 cities with the most
skyscrapers in the world.
How to deliver these superstructures is
an interesting challenge for the construction
industry. As such, there’s been a dramatic
shift in the way that buildings are designed.
Traditionally, tall buildings were considered
anything up to 12 storeys. Now the challenge
is to design and construct buildings 40
storeys tall and above.
From the perspective of concrete, this
meant we previously needed to deliver
a compressive strength of up to 60MPa.
However, the demands of tall building
construction today are such that structures
need significantly higher compressive
Thevalueof
specialconcretes
intallbuildings
IgnacioEscobarMIStructE,NationalEngineeringManager,
TarmacReadymix,UK
Synopsis
In recent years, a shortage of development space in many
cities has led to a rise in the construction of tall buildings.
This article looks at the role special concrete mixes – high-
early-strength concrete and self-compacting concrete
– have to play in the design and construction of medium-rise
and tall buildings. Using research case studies, the article
considers the potential savings in construction costs offered
by the use of special concretes, as well as added value that
may be realised through an increase in floor area.
TARMAC
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2. 21
www.thestructuralengineer.org
strengths – as high as 90MPa – so that
we can meet developers’ demands. High-
strength concrete is now specified for the
vertical elements of many new tall buildings
to satisfy more challenging load path,
robustness and stability criteria.
Eurocode 22
, which relates to the
design of concrete structures and was
approved by the European Committee for
Standardisation in 2004, introduced the use
of higher compressive strengths in the UK:
up to C90/105. Strengths had previously
been limited to C50/60 by BS 8110. This
change considerably improved the long-
term deflection control and performance of
structural elements in general.
But there are other issues that have been
overlooked. We continue to design elements
such as floors to 28 days for concrete to
reach its optimum strength. However, with
the development of high-early-strength
concretes, engineers could benefit from
higher mechanical properties at early days
of the concrete and high strengths at 28
days. Too often, special concretes (Box 1) are
used only if and when a project falls behind
schedule. But that doesn’t need to be the
case, as if they’re specified from the outset,
these products could have a positive effect
on the overall build programme.
We should be adopting a holistic approach
when specifying building materials, examining
their effect on the overall build timescales
rather than considering each constituent
material in isolation. Take concrete: high-
strength and high-early-strength concretes
can be used for more than just the vertical
elements of a new build. For example, there
are significant savings to be had from using
special concretes for floors too.
We’re now in a position where long-term
deflections can be controlled by not only
using span-to-depth ratios, but by assessing
the theoretical deflections using the
expressions available in section 7.4.3 of EC2.
There has been a significant shift since the
introduction of Eurocodes in the UK, because
EC2 provides deflection calculations that are
more advanced than span-to-depth ratios
and BS 8110, as it considers the effects of
early-age construction loading and high early
tensile strengths in crack formation.
Using high-strength concrete allows
long-term deflections to be controlled while
keeping floor thickness to a minimum. Using
the same product for both the floors and
walls also reduces the risk of differential
stiffness between vertical and horizontal
elements.
This also has an important role to play in
reducing the weight of the building. Laying
thinner material for both the walls and floors
not only has a beneficial outcome on the
available space and light in the subsequent
rooms, but also makes the building weigh
less. This means that the building needs
smaller foundations and a reduced facade
surface, resulting in cost savings because
less material is needed for construction than
when using traditional concrete.
Research
There is a step-change opportunity for the
construction industry to move away from
using conventional concrete and deliver value
through adopting special concrete solutions.
To support this notion, Tarmac commissioned
Arup to carry out independent research
"Howtodeliverthesesuperstructures
isaninterestingchallengeforthe
constructionindustry.Assuch,there’s
beenadramaticshiftinthewaythat
buildingsaredesigned"
reviewing both in situ concrete and composite
steel-framed multistorey building structures.
The intention was to identify where two
concrete innovations (self-compacting and
high-early-strength concretes) can add value
in both types of construction for residential
and commercial buildings.
Arup selected case study buildings from
its recent projects to take advantage of
factual information and be representative of
real-world construction. Arup reviewed the
potential design benefits of reduced rebar or
member sizes and also the associated labour
and programme advantages.
High-strength concrete is a concrete that
develops significant compressive strength
soon after it has been poured, typically
35–40MPa at 24–72 hours. This concrete
is used when a fast rotation of formwork is
desired or when post-tensioning is used.
The benefits of using this concrete are the
increased speed of construction and the
availability of the structural elements to be
loaded without having to wait until its
28-day strength.
Self-compacting concrete has many
benefits over conventional compacted
concrete. Highly fluid, it can be poured
quickly and easily consolidates into the
desired area, eliminating the need for
power tools, reducing noise and vibration
during construction. This enables fast
placement, requiring fewer pour points,
less formwork and reduced labour,
and results in a smooth surface quality,
eliminating the need for floating. It can be
placed using typically a third of the labour
and a quarter of the time compared to a
conventional mix.
Screed products can have a significant
impact on the overall build schedule due
to the drying time required. However,
certain materials are available that dry in
a quarter of the time of traditional floor
screed. These gift any build schedule a
great deal of flexibility, as they are not
only significantly quicker to lay than
other materials, but the drying time is
reduced to just 14–21 days, as opposed to
40–75 days for other traditional or flowing
screed products. They can also take foot
traffic after just 24–48 hours, meaning
floor coverings can be applied sooner.
Employing products like these can have
tangible benefits for the overall schedule
and make cost savings from a reduction
in labour and plant equipment, as well as
enabling the building to be let quicker.
Box1.Whatarespecialconcretes?
Table 1: Cost savings in composite slab and shear core construction
Core £1 772 867 £1 602 234 –9.6%
Composite slabs £11 967 701 £11 870 557 –0.8%
Total £13 740 568 £13 472 791 –1.9%
Element Cost of base case Cost incorporating
special mixes
Change
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3. 22 TheStructuralEngineer Feature
›
September2016 Special concretes in tall buildings
Box2.NineElmsdevelopment–residentialbasecasestudy
Two case study buildings were considered:
a 16-storey reinforced-concrete flat-slab
residential building from the Nine Elms
development in London (Box 2) and a
21-storey steel-framed commercial building
with reinforced-concrete cores and
composite concrete slabs, at Ropemaker
Place, London (Box 3). These buildings
were selected as representative of typical
UK developments of medium height (10–25
floors). This is tall enough that benefits
relevant to towers will be highlighted, but also
short enough that super-tall tower effects
do not govern the design; hence, the results
will also be relevant to mixed-use low- and
medium-rise structures.
Results: steel-framed buildings
The review for steel-framed buildings was
undertaken at an element level of walls and
composite slabs. The results indicated that,
for a commercial building, there are significant
programme savings for composite slabs and
shear cores.
As self-compacting concrete is assumed to
have the same structural design properties as
a standard concrete mix, there are no material
quantity reductions to be achieved. However,
labour savings are achieved as the material
can be poured more quickly and requires less
finishing than a standard mix. For the case
study in question, this represents an overall
cost saving of £267 777, equating to 1.9%.
Faster construction also allows earlier
access for follow-on trades, both above and
beneath the slab. These cost savings are
shown in Table 1 (negative figures represent
savings).
Furthermore, self-compacting concrete can
be designed to provide high early strengths,
with associated material quantity savings.
Results: concrete buildings
The review for concrete buildings was
undertaken at an element level of columns,
slabs and walls. Table 2 shows the cost
savings for structural elements only in the
concrete-framed case study. Combining
these material, plant and labour savings
with the reduced facade area gives a total
saving of £391 722. Considering an estimated
concrete package cost of £4 650 893 for the
base case building, this represents a saving
of 8.4% with respect to the concrete frame
construction cost.
(NB Values relate to construction costs in
2015 based on market testing with the supply
chain, use of Arup’s internal data and Spon’s
Civil Engineering and Highway Works Price
Book 20153
.)
Results: overall savings and added value
Table 3 summarises the overall cost savings
and value added to the buildings considered
Table 2: Cost savings for structural elements in concrete-framed case study
Cores £737 848 £697 372 –5.5%
Columns £222 774 £215 510 –3.3%
£3 372 214 £3 112 974 –7.7%
£318 057 £303 832 –4.5%
Total £4 650 893 £4 329 689 –6.9%
Element Cost of base case Cost incorporating
special mixes
Change
Post-tensioned
slabs
Reinforced-
concrete slabs
by Arup in the study. The cost savings are
mainly due to reduced material quantity
and labour required when using high-early-
strength concrete, with a subsequent
reduction in the required facade area; and
reduced labour and plant needed when using
self-compacting concrete.
The value added was due to additional
saleable area resulting from the reduction
of vertical element sizes. For the concrete-
AVRLONDON
The Nine Elms development
on London’s South Bank is a
regeneration project for the
boroughs of Lambeth and
Wandsworth, offering 20 000
new homes, 25 000 new jobs,
new schools, parks, culture and
the arts. At its heart is Embassy
Gardens, residential buildings
inspired by commercial buildings in
Manhattan’s meatpacking district
in the 19th century, constructed
from a series of combined
blocks with garden terraces. The
development is being delivered
by a collaboration between
Ballymore and EcoWorld.
Source: http://nineelmslondon.com
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4. 23
www.thestructuralengineer.org
References
E1 New London Architecture and GL Hearn (2015) London Tall Buildings Survey
2015 [Online] Available at: www.newlondonarchitecture.org/docs/tall_
bldgs_survey_2015.pdf (Accessed: April 2016)
E2 British Standards Institution (2004) BS EN 1992-1-1:2004+A1:2014 Eurocode
2: Design of concrete structures. General rules and rules for buildings,
London, UK: BSI
E3 AECOM (ed.) (2015) Spon’s Civil Engineering and Highway Works Price Book
2015 (29th ed.), Boca Raton, USA: CRC Press
Box3.RopemakerPlace–commercialbasecasestudy
Table 3: Summary of results from building study
£391 722 £175 000
£267 777 £69 400
Concrete-framed
residential building
Composite steel-
and-concrete-framed
office building
Including facade cost saving,
8.4% reduction in construction
cost with respect to cost of
structural frame
1.9% reduction in construction
cost with respect to cost of
structural frame
Increased sale price due to
additional floor area
Cost savings Value added
Increased rental revenue
per year due to additional
floor area
framed residential building, this equated to
£175 000 from an increased sale price due to
additional floor area. For the composite steel-
and-concrete-framed office building, this
equated to £69 400, achieved from increased
rental revenue per year due to additional floor
area.2
Conclusions
The findings indicate that further programme
reductions could be achieved if the concrete
cores for the case study buildings had been
on the critical path and were constructed
using jump forms. Using high-early-
strength concrete in this way can
potentially offer a 40% time saving for
the core construction, translating into a
5% reduction in the total programme.
Ultimately, office and residential
buildings could be let or sold quicker,
so the building owner can recover
costs more quickly. Overall, there
is also less disruption to the local
community from a reduced build
programme.
These results indicate that
appropriate consideration of special
concretes at the design and build
stages can be beneficial and that
most benefit is gained if the designer
includes the benefit of both early strength
and high 28-day strength.
Additional benefits that weren’t
specifically covered in the study but are
applicable to special concretes are in
the field of health and safety. With many
tall buildings being constructed within
constrained sites in busy city environments,
the footprint of a building is often the entire
site. Reducing the volumes of material
needed to complete the large pours reduces
deliveries to site, meaning less traffic in
already highly congested cities. Health
and safety is improved as fewer people
are needed for the pour and there is no
need for compacting tools, which not only
eases placement but reduces noise levels;
essential for high-density urban areas.
When concrete is on the critical path,
its specification is critical to cut build
timescales, maximise space and ultimately
deliver financial savings. The industry should
be looking at the bigger picture, the lifecycle
of the build and not just individual product
costs in isolation. We need to break down
the barrier of tried-and-tested methods and
welcome innovation.
Boundaries are being pushed further
than ever before, forging a skyline that
showcases tall buildings. Architects are
designing smarter; we need to ensure
that we’re building smarter so that we can
meet this challenge of constructing taller
buildings for high-density urban areas.
Ropemaker Place is a 21-storey, 83 710m2
tower positioned
between Moorgate and Islington in east London, designed by
Arup Associates in 2009 for The British Land Company PLC.
The brief was to create an inspiring, impressive and sustainable
building, in no small part achieved by using 24% construction
materials from recycled sources with 16% of the occupier’s
energy demand met by on-site renewables. The tower takes the
form of six large-scale interlocking cubic forms, designed as
a “simplified Chinese puzzle”, with a glass facade to maximise
natural light in the office spaces.
Source: http://arupassociates.com/en/case-studies/ropemaker
ALAMY
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